Questions tagged [branch-prediction]

In computer architecture, a branch predictor is a digital circuit that tries to guess which way a branch (e.g. an if-then-else structure) will go before this is known for sure. The purpose of the branch predictor is to improve the flow in the instruction pipeline. Branch predictors play a critical role in achieving high effective performance in many modern pipelined microprocessor architectures such as x86.

Why is it faster to process a sorted array than an unsorted array? Stack Overflow's highest-voted question and answer is a good introduction to the subject.

In computer architecture, a branch predictor is a digital circuit that tries to guess which way a branch (e.g. an if-then-else structure) will go before this is known for sure. The purpose of the branch predictor is to improve the flow in the instruction pipeline.

Branch predictors play a critical role in achieving high effective performance in many modern pipelined microprocessor architectures such as x86.

Two-way branching is usually implemented with a conditional jump instruction. A conditional jump can either be "not taken" and continue execution with the first branch of code which follows immediately after the conditional jump - or it can be "taken" and jump to a different place in program memory where the second branch of code is stored.

It is not known for certain whether a conditional jump will be taken or not taken until the condition has been calculated and the conditional jump has passed the execution stage in the instruction pipeline.

Without branch prediction, the processor would have to wait until the conditional jump instruction has passed the execute stage before the next instruction can enter the fetch stage in the pipeline. The branch predictor attempts to avoid this waste of time by trying to guess whether the conditional jump is most likely to be taken or not taken. The branch that is guessed to be the most likely is then fetched and speculatively executed. If it is later detected that the guess was wrong then the speculatively executed or partially executed instructions are discarded and the pipeline starts over with the correct branch, incurring a delay.

The time that is wasted in case of a branch misprediction is equal to the number of stages in the pipeline from the fetch stage to the execute stage. Modern microprocessors tend to have quite long pipelines so that the misprediction delay is between 10 and 20 clock cycles. The longer the pipeline the greater the need for a good branch predictor.


The Spectre security vulnerability revolves around branch prediction:

Other resources

Special-purpose predictors: Return Address Stack for call/ret. ret is effectively an indirect branch, setting program-counter = return address. This would be hard to predict on its own, but calls are normally made with a special instruction so modern CPUs can match call/ret pairs with an internal stack.

Computer architecture details about branch prediction / speculative execution, and its effects on pipelined CPUs

  • Why is it faster to process a sorted array than an unsorted array?
  • Branch prediction - Dan Luu's article on branch prediction, adapted from a talk. With diagrams. Good introduction to why it's needed, and some basic implementations used in early CPUs, building up to more complicated predictors. And at the end, a link to TAGE branch predictors used on modern Intel CPUs. (Too complicated for that article to explain, though!)
  • Slow jmp-instruction - even unconditional direct jumps (like x86's jmp) need to be predicted, to avoid stalls in the very first stage of the pipeline: fetching blocks of machine code from I-cache. After fetching one block, you need to know which block to fetch next, before (or at best in parallel with) decoding the block you just fetched. A large sequence of jmp next_instruction will overwhelm branch prediction and expose the cost of misprediction in this part of the pipeline. (Many high-end modern CPUs have a queue after fetch before decode, to hide bubbles, so some blocks of non-branchy code can allow the queue to refill.)
  • Branch target prediction in conjunction with branch prediction?
  • What branch misprediction does the Branch Target Buffer detect?

Cost of a branch miss

Modern TAGE predictors (in Intel CPUs for example) can "learn" amazingly long patterns, because they index based on past branch history. (So the same branch can get different predictions depending on the path leading up to it. A single branch can have its prediction data scattered over many bits in the branch predictor table). This goes a long way to solving the problem of indirect branches in an interpreter almost always mispredicting (X86 prefetching optimizations: "computed goto" threaded code and Branch prediction and the performance of interpreters — Don't trust folklore), or for example a binary search on the same data with the same input can be really efficient.

Static branch prediction on newer Intel processors - according to experimental evidence, it appears Nehalem and earlier do sometimes use static prediction at some point in the pipeline (backwards branches default to predicted-taken, forward to not-taken.) But Sandybridge and newer seem to be always dynamic based on some history, whether it's from this branch or one that aliases it. Why did Intel change the static branch prediction mechanism over these years?

Cases where TAGE does "amazingly" well

Assembly code layout: not so much for branch prediction, but because not-taken branches are easier on the front-end than taken branches. Better I-cache code density if the fast-path is just a straight line, and taken branches mean the part of a fetch block after the branch isn't useful.

Superscalar CPUs fetch code in blocks, e.g. aligned 16 byte blocks, containing multiple instructions. In non-branching code, including not-taken conditional branches, all of those bytes are useful instruction bytes.

Branchless code: using cmov or other tricks to avoid branches

This is the asm equivalent of replacing if (c) a=b; with a = c ? b : a;. If b doesn't have side-effects, and a isn't a potentially-shared memory location, compilers can do "if-conversion" to do the conditional with a data dependency on c instead of a control dependency.

(C compilers can't introduce a non-atomic read/write: that could step on another thread's modification of the variable. Writing your code as always rewriting a value tells compilers that it's safe, which sometimes enables auto-vectorization: AVX-512 and Branching)

Potential downside to cmov in scalar code: the data dependency can become part of a loop-carried dependency chain and become a bottleneck, while branch prediction + speculative execution hide the latency of control dependencies. The branchless data dependency isn't predicted or speculated, which makes it good for unpredictable cases, but potentially bad otherwise.

363 questions
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Why is processing a sorted array faster than processing an unsorted array?

In this C++ code, sorting the data (before the timed region) makes the primary loop ~6x faster: #include #include #include int main() { // Generate data const unsigned arraySize = 32768; int…
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How do the likely/unlikely macros in the Linux kernel work and what is their benefit?

I've been digging through some parts of the Linux kernel, and found calls like this: if (unlikely(fd < 0)) { /* Do something */ } or if (likely(!err)) { /* Do something */ } I've found the definition of them: #define likely(x) …
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Why is (a*b != 0) faster than (a != 0 && b != 0) in Java?

I'm writing some code in Java where, at some point, the flow of the program is determined by whether two int variables, "a" and "b", are non-zero (note: a and b are never negative, and never within integer overflow range). I can evaluate it with if…
10 answers

What is the effect of ordering if...else if statements by probability?

Specifically, if I have a series of if...else if statements, and I somehow know beforehand the relative probability that each statement will evaluate to true, how much difference in execution time does it make to sort them in order of probability?…
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Why is processing an unsorted array the same speed as processing a sorted array with modern x86-64 clang?

I discovered this popular ~9-year-old SO question and decided to double-check its outcomes. So, I have AMD Ryzen 9 5950X, clang++ 10 and Linux, I copy-pasted code from the question and here is what I got: Sorted - 0.549702s: ~/d/so_sorting_faster$…
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An expensive jump with GCC 5.4.0

I had a function which looked like this (showing only the important part): double CompareShifted(const std::vector& l, const std::vector &curr, int shift, int shiftY) { ... for(std::size_t i=std::max(0,-shift);i
Jakub Jůza
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Is there a compiler hint for GCC to force branch prediction to always go a certain way?

For the Intel architectures, is there a way to instruct the GCC compiler to generate code that always forces branch prediction a particular way in my code? Does the Intel hardware even support this? What about other compilers or hardwares? I would…
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Is "IF" expensive?

I can't, for the life of me, remember what exactly our teacher said that day and I'm hoping you would probably know. The module is "Data Structures and Algorithms" and he told us something along the lines of: The if statement is the most expensive …
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Why is a conditional move not vulnerable to Branch Prediction Failure?

After reading this post (answer on StackOverflow) (at the optimization section), I was wondering why conditional moves are not vulnerable for Branch Prediction Failure. I found on an article on cond moves here (PDF by AMD). Also there, they claim…
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In Java, can & be faster than &&?

In this code: if (value >= x && value <= y) { when value >= x and value <= y are as likely true as false with no particular pattern, would using the & operator be faster than using &&? Specifically, I am thinking about how && lazily evaluates the…
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What does `rep ret` mean?

I was testing some code on Visual Studio 2008 and noticed security_cookie. I can understand the point of it, but I don't understand what the purpose of this instruction is. rep ret /* REP to avoid AMD branch prediction penalty */ Of course I…
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Is if(A | B) always faster than if(A || B)?

I am reading this book by Fedor Pikus and he has some very very interesting examples which for me were a surprise. Particularly this benchmark caught me, where the only difference is that in one of them we use || in if and in another we use |. void…
Eduard Rostomyan
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When should streams be preferred over traditional loops for best performance? Do streams take advantage of branch-prediction?

I just read about Branch-Prediction and wanted to try how this works with Java 8 Streams. However the performance with Streams is always turning out to be worse than traditional loops. int totalSize = 32768; int filterValue = 1280; int[] array =…
Kishore Bandi
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Portable branch prediction hints

Is there any portable way of doing branch prediction hints? Consider the following example: if (unlikely_condition) { /* ..A.. */ } else { /* ..B.. */ } Is this any different than doing: if (!unlikely_condition) { /* ..B.. */ …
Oskar N.
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Branch-aware programming

I'm reading around that branch misprediction can be a hot bottleneck for the performance of an application. As I can see, people often show assembly code that unveil the problem and state that programmers usually can predict where a branch could go…
Paolo M
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